1. Field of the Invention
The present invention relates to a construction machine such as a hydraulic excavator.
2. Description of the Related Art
An example of a conventional hydraulic excavator will be described below with reference to FIG. 4 which is a side view thereof.
In the same figure, the reference numeral 30 denotes a hydraulic excavator. The hydraulic excavator 30 has a lower carriage 31. The lower carriage 31 is provided with a pair of crawlers 31a, the crawlers 31a each comprising a track frame 31b, an idler wheel 31c and a traveling motor 31d which are mounted through shafts at front and rear ends, respectively, of the track frame 31b, and a shoe 31e entrained on both idler wheel 31c and traveling motor 31d. The paired crawlers 31a are connected together through a center frame (not shown).
On top of the lower carriage 31 is mounted a rotatable superstructure 32. The rotatable superstructure 32 is provided with a counter weight 32a mounted at the rear end thereof and a cab 37 formed at the front portion thereof. The cab 37 is provided with an operator seat (not shown) disposed at a rear position in the interior of the cab, a pair of operating levers (not shown) disposed at both side positions in front of the operator seat, and a pair of traveling levers (not shown) disposed in front of the operator seat.
In front of the cab 37 is provided an attachment 33 so that it can rise and fall with a boom foot pin (not shown) as fulcrum, the attachment 33 comprising a boom 34, an arm 35 and a bucket 36. The boom 34 is made capable of rise and fall by means of a boom cylinder 34a both ends of which are connected respectively to the front end of rotatable superstructure 32 and the boom 34. The arm 35 is connected pivotably to the front end of the boom 34. The arm 35 is made pivotable by means of an arm cylinder 35a which is disposed between the back of the boom 34 and a base end of the arm 35. The bucket 36 is mounted pivotably at a front end portion of the arm 35. The bucket 36 is made pivotable by means of a bucket cylinder 36a disposed between the bucket and the back of the arm 35.
An operator of the hydraulic excavator sits on the operator seat and operates the traveling levers to supply a hydraulic oil to each traveling motor 31d from a hydraulic pump which is mounted in the interior of the rotatable superstructure and which will be described later, thereby causing movement of the hydraulic excavator. Likewise, the operator operates the operating levers to supply the hydraulic oil from the hydraulic pump to a rotating motor which will be described later, thereby causing rotation of the rotatable superstructure. Further, the hydraulic oil is fed to the cylinders 34a, 35a and 36a to actuate the attachment 33, thereby performing operations such as excavation.
The above conventional hydraulic excavator is provided with two such hydraulic pumps as referred to above. By operation of a control valve, for example during work, a first pump is used for the boom cylinder 34a and the bucket cylinder 36a and a second pump is used for the arm cylinder 35a and the rotating motor (not shown). On the other hand, during traveling, the first and second pumps are used for a right traveling motor (31d) and a left traveling motor (31d), respectively. This state is assumed to indicate that the control valve is in the position of neutral function.
Further, in the case where the traveling motors 31d and any (hereinafter referred to as the "work machine actuator") of the boom cylinder 34a, bucket cylinder 36a, arm cylinder 35a and rotating motor (not shown) are driven at a time, the first and second pumps, for example, are used exclusively for the work machine actuator and the traveling pump, respectively. This state is assumed to indicate that the control valve is in the position of independent traveling function.
However, even when the work machine actuator is driven while both right and left traveling motors are in operation, the control valve switches from the position of neutral function to the position of independent traveling function, so that switching is made from the previous oil distribution to the right and left traveling motors by the first and second pumps respectively into the oil distribution to both right and left traveling motors by only the second pump. Consequently, the load on the second pump doubles and the flow rate is reduced by half, thus giving rise to a deceleration shock.
In general, therefore, the independent traveling function of the control valve is changed into a straight traveling function while providing a pump communication path for communication between the first and second pumps. By so doing, even where the traveling motors and the work machine actuator are driven at a time, the oil present in the first pump is distributed to the second pump, whereby a shock such as a deceleration shock is cushioned to a certain extent.
However, the following problem has been encountered in the aforesaid change from the independent traveling function of the control valve to the straight traveling function.
When the work machine actuator is to be operated while the hydraulic excavator is moving up a steep slope (say at a second pump pressure of 300 k) and when the work which is done by the work machine actuator is under no load or under a light load (say at a first pump pressure of 100 k), the oil present on the second pump side flows to the first pump side through the first-second pump communication path in the straight traveling function, with the result that the traveling of the excavator stops and the work machine accelerates.
Conversely, when the boom as a work machine is operated in its rising direction in a state in which the traveling motor pressure is not so high (say at a second pump pressure of 100 k), for example during a load lifting work on a level ground, the oil present on the first pump side flows to the second pump side by the straight traveling function because of a high boom rising load pressure (say 200 k), so that the traveling speed increases and the motion of the work machine becomes slow or stops.
More particularly, with the control valve assuming the position of straight traveling function, when the hydraulic pressure on either the traveling side or the work machine side becomes high, oil flows to the low pressure side through the first-second pump communication path in the straight traveling function, thus giving rise to the problem that the high-pressure side becomes inoperative or the low-pressure side accelerates.